NEW YORK — Steve Martin was in his Gray Ghost mode — polite, but distant and a bit out of focus — at the April Fool’s Day benefit bash at Union Square’s W Hotel. The occasion was the performance of his fast-and-loose adaptation of Carl Sternheim’s 1910 German farce “The Underpants” at the Classic Stage Co.Underdressed guests — among them Kevin Kline, Phoebe Cates, Wendy Wassterstein and Nora Ephron — milled about the bar, picking through silent auction items like a Guggenheim Museum membership and tickets to a Knicks game. Martin was all the while mulling his contributions to sex farce. One of his embellishments to Sternheim, he said, was giving a heightened sense of awareness to the flighty young hausfrau who achieves, and then loses, her 15-minutes of notoriety when she drops her panties in public. “That’s the sad part of being famous,” he said, “when the moment is past and the person is left without her glory.” It was, he said, “a little poignant.” And did he ever feel that loss of glory himself? “Oh, maybe back in the 1980s,” he chuckled.